Hair Today, Gone Tomorrow

There's more to a winter coat than shedding aggravation. If we want to better manage our horses' winter coats, first we need to understand what they are and how they grow.

Photo: Photos.com

When it comes to winter coats, most of us think of the long hairs that end up down our shirts, up our noses, in our mouths and eyes, and all over the floor during springtime shedding. However, there's more to a winter coat than shedding aggravation. If we want to better manage our horses' winter coats, first we need to understand what they are and how they grow.

Hair functions as an important structure involved in thermal regulation, sensory perception, and as a barrier to chemical, physical, and microbial injury to the skin. The discussion of winter coats for horses must start with skin and hair growth/ composition in general. As we will see, the cycle of hair growth is complicated and influenced by multiple factors.

The skin is obviously the structure containing the hair follicles and is actually considered a body organ. A horse's skin averages almost one inch in thickness, but varies over different body areas from as thin as less than one-half inch in areas such as the head and underbelly to 1 1/2 inches thick in areas such as the lower back and rump. Along with the hair follicles, the skin contains millions of sensory nerve cells and sweat glands, and given its huge surface area (approximately 7,750 square inches for a 1,000-pound horse), it plays an important role in controlling body temperature (thermoregulation).

The skin is also the primary barrier between the outside world and the internal structures of the body, and it provides protection from dehydration and infection. The skin actually has a normal population of bacteria that live in balance (the normal bacterial "flora") and act as competition to potential invading bacteria.

Hair Coat

A horse's hair coat changes with the seasons; therefore, the ability of the hair coat to regulate body temperature is related to its length, thickness, and density per square inch of surface area of skin. One major factor in the hair's ability to serve in thermoregulation is the ability of a small muscle associated with every hair follicle under control of the nervous system to pull the hairs to a standing "puffed-up" position (piloerection). The physiological process of piloerection increases the insulating factor of the hair coat by increasing the air content within the hair coat and therefore the skin and the environment. It's like putting on a down vest. Some animals also enjoy other benefits of piloerection; for example, your barn cats might fluff up to make themselves look bigger and more intimidating to your barn dogs.

A thought on blankets at this point: If the horse has a normal full winter hair coat, blankets that fit too tightly or are very heavy might actually prevent the hair from raising and therefore reduce the natural insulating effect should the horse need it. There are essentially two layers of hair that make up the coat. There is a primary coat known as the outer or "guard" coat, and a secondary coat known as the inner coat. It has been reported that there are approximately 800-1,200 primary hairs and from 1,200-2,000 secondary hairs per square inch of skin in the horse.

Hair grows from the follicle structure within the skin. If the follicle is straight, the hair grows straight, and if the follicle is slanted within the skin, the hair grows slanted. As we all know, the hair coat for the most part has a typical orientation pattern over the horse's body, with the exception of often-unique whorls, cow-licks, etc. One additional aspect to note is that the hair coat of the adult horse is typically quite different from the foal. The changes are speculated to reflect different requirements for heat regulation, camouflage, and sexual/social communication.

From an evolutionary point of view, the effects of domestication on the genes controlling hair coat presently are quite young compared to the genetic programming of thousands and perhaps millions of years. For example, if you look at the mountain and moorland ponies that live outdoors in the United Kingdom, they are considered to have five distinct hair coats during their lifetimes--birth, foal, yearling, adult summer, and adult winter.

Hair does not grow continuously, but in cycles. There is a growing cycle (called the anagen phase), when the follicle is actively growing a hair, and a resting cycle (called the telogen phase), when the produced hair is retained within the follicle during the cycle, but is actually a dead hair and will subsequently be lost or shed. There is also a transition phase between the growth and resting cycles.

A final note on hair growth: Hair is composed mainly of protein and obviously requires energy to be manufactured by the body, so poor nutritional states can have a significant effect on both the quantity and quality of hair growth. Also, generalized systemic illness can decrease the quality of a hair coat.

Getting Ready for Winter

Now we have some basic understanding of hair, and things are starting to get a little chilly out at night (at least in my part of the country--New York). The big northeast indoor shows are around the corner, and the one light sheet has become two, then three, then you add one blanket or two to top it all off at night in effort to put a stop to the winter coat from coming on strong. Despite the clothing, most horses will still need to be body clipped before the Washington National.

The major player in the hair coat cycles appears to be the changing length of daylight or photoperiod. The daily photoperiod effect on hair growth cycles brings the brain into the act. Light signals are routed biochemically to the pineal gland (an area deep within the brain once believed by Descartes to be the "seat of the soul"), the hypothalamic part of the brain, and the pituitary gland. From there the control is hormonal (chemicals released into the bloodstream by one structure affect another structure) via changes in hormones produced by the pituitary gland, thyroid gland, adrenal gland, and even the ovaries and testicles (if they have not been removed). It has been shown that castration does not have a direct effect on hair growth.

Skin temperature does play a role, and it has been shown that blanketing and/or heated stabling can stop a full winter coat from coming in, but it is not an on-off switch alone.

Illness and Hair Coats

Disease states can affect on hair growth. For example, 75% of horses with a pituitary tumor (Cushing's disease) will have excessively long, curly hairs with reduced or no shedding resulting from the hormonal changes related to the tumor. Many of these horses are suspected of--and are treated for--low thyroid function (hypothyroidism), which is another systemic disease that can affect the hair coat. In dogs, there is a very classic loss of hair (alopecia) over the large parts of the body associated with the loss of thyroid function. There are several reports of naturally occurring hypothyroidism in horses with similar hair loss.

Assessing thyroid function in the horse is complicated by the fact that the baseline measurements of the thyroid hormones can be low in horses with normally functioning thyroid glands because of other illnesses and conditions. The definitive diagnosis of hypothyroidism requires the administration of a thyroid stimulating hormone (TSH) and subsequent measurement of the thyroid hormones, looking for a failure of thyroid hormone levels to rise.

Remember that if your horse is stressed by a physical problem, you should make sure his environment helps the cause, not hurts it. Don't blanket him so heavily that he is dripping sweat, and don't throw him out into the cold if an illness has debilitated his ability to generate adequate body heat.

Also, don't forget a daily check on your turned-out horses wearing blankets. They can become thin or develop rubs or problems under the blankets that you might not notice without regularly removing the blankets and running your hands all over their bodies.

Spring Shedding

Then comes spring--the daylight lengthens, the air warms, and the hair starts to fly. It has been noted that Exmoor, moorland, and mountain ponies living in the UK (and the Przewalski horses living in the area of the former Czechoslovakia) shed only once a year between March and May, and the shed can take as long as seven weeks. It is also noted that a cold, wet season can delay the shed and that the starting date of the shed varies from horse to horse within the same year as well as varying from year to year on the same horse. The summer coat lasts from June to August in these horses, with hair growth increasing in the autumn (both in hair length and density of the coat). The winter coat typically lasts from September to May.

It has been shown that horses wearing blankets and/or stabled in heated barns fail to develop a complete or "full" winter coat. It has been hypothesized that domestication (heated barns, blanketing, and rapid changes in geographic location) and breeding schedules (hormones out of sync with Mother Nature's schedule and mares standing under artificial light) might cause a change from a single yearly shed as observed in the undomesticated horse, to continuously shedding throughout the year, maintaining a short coat.

On a final note, as the season of blankets and heated barns creeps up on us again, be careful not to get your horse in an unhealthy situation. The heating of barns within reason is fine, but not at the expense of adequate ventilation and fire safety. Also keep in mind that just because the air quality seems fine to you out in the barn aisle, it might not be all that good in a confined stall closer to ground level where your horse's nose (and a newborn foal's nose) spends most of its time. Inattention to adequate ventilation in heated barns (and sometimes even in non-heated barns) can contribute to respiratory problems.

Another area involving hair coats to keep in mind is shipping. It is amazing how much body heat even two heavily blanketed horses (let alone 10-12) can generate within even a partially closed horse trailer. Stop after 20-30 minutes on the road and check out the situation in the trailer. The inside trailer temperature is also drastically different between night and day as the sun (if you are lucky enough to see some sun during the winter months) rises and starts to heat the trailer. Also, for the lucky horses heading south or west from the northeast for the winter, keep an eye on the outside air temperature and adjust the blankets/ windows/vents accordingly, which holds true anytime you are traveling with your horses.

I have found horses dripping wet with sweat and overheated when too heavily blanketed with all the windows/vents closed down. Again, the ventilation is important, so make an effort to balance some degree of indirect ventilation and the degree of clothing for the given the circumstances of the day/night and individual shipping conditions.

In summary, winter coats might be an annoyance, but they evolved for a reason. Understand how they work and what makes them grow, and you can manage them according to your situation to keep your horse comfortable throughout the seasons.

ADDITIONAL READING:

Scott, D.; Miller, W. Equine Dermatology. St. Louis: Saunders, 2003.

About the Author

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

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